Most of the world production of rice is consumed as white rice with the hull and bran layer removed. The bran layer makes up about 8 to 10% of the rough rice weight and is rich in protein, lipids, certain vitamins and trace minerals. Recent studies have even indicated that a diet supplemented with rice bran may be an effective means of reducing serum cholesterol in humans. There is a great abundance of rice bran and it is an important source of high quality cooking oil, however, it is considered in most countries a by-product and is disposed of or immediately sold as animal feed. In spite of its being rich in protein, lipids and certain vitamins, several obstacles have hindered its utilization.
Rice bran has a high oil content of 15-29%, depending on milling procedures and hull contamination. Because of the high oil content, naturally present enzymes or enzymes produced as a result of microbiological activity, hydrolyzes the oil and cause it to become rancid. Rancidity in rice bran causes it to have a bitter and soapy taste. Since rancidity occurs very rapidly at room temperature, rice bran is used mainly as a high protein feed additive for feedstock.
In order to extent the shelf life of rice bran from rough or paddy rice for later consumption, it must be stabilized immediately after milling to minimize its free fatty acid (FFA) content. Studies have repeatedly shown that free fatty acids develop rapidly in untreated rice bran or loosely milled rice during the first few days or weeks after milling. This change profoundly affects the value of bran for the extraction of oil. As the FFA content rises, oil-refining losses increase proportionately. While oxidative changes in the rice bran also negatively impact oil quality, these changes are not as rapid or obvious. Therefore, primary attention has been focused on stabilization efforts directed to the destruction or inhibition of lipase, the enzyme responsible for FFA development, rather than to the auto-oxidative changes. Heat stabilization has been used to either reversibly inhibit or permanently denature the lipase enzyme that is primarily responsible for the hydrolytic degradation of the oil in the bran. Other attempts to stabilize rice bran have included dry heat, wet heat and extrusion methods.
Although hydrolytic rancidity can be controlled through extrusion stabilization, methods for controlling oxidative rancidity, which develops over longer periods of storage times, have not been found. Oxidative stability is dependent on endogenous antioxidant compounds such as tocopherols and oryzanols. Oxidative deterioration of fats generally occurs by a free radical mechanism. In the initiation step, an active hydrogen, especially in the presence of a metal catalyst, such as copper, is removed from a triglyceride to yield a free radical. The free radical can then combine with oxygen to form a peroxide-free radical, which removes hydrogen from another unsaturated molecule to yield a peroxide and a new free radical. This propagation stage becomes a chain reaction and may continue until the free radicals react with each other to form inactive products, leading to termination of the cycle.
Peroxides are the primary oxidation products. Peroxides are quite unstable and decompose into a range of secondary products, including aldehydes, alcohols and ketones, which produce the typical rancid oil aroma. It is only during the initial stages of oxidative deterioration that the peroxide value may be used to indicate oxidative deterioration. Peroxide value (PV) is a term used to qualify the content, expressed in milli-equivalents of peroxide per kilogram of sample (meq/kg), of all substances that oxidize potassium iodide under specified conditions.
Rice can be milled in the rough or paddy state or it can be parboiled prior to milling. Bran from rice that has been parboiled has been shown to exhibit a reduced level of FFA, as compared to unprocessed rice bran and to be more resistant to the development of FFA during storage. It is generally accepted that lipase enzymes are destroyed in the parboiling process, due to the treatment times and temperatures typically involved in parboiling. It appears that some pre-formed free fatty acids are apparently leached out, oxidized and/or complexed with starch, which accounts for the initial reduction in FFA in parboiled rice bran. However, the oil in parboiled rice and in bran from parboiled rice has been shown to be highly susceptible to oxidative deterioration. This is generally attributed to destruction or removal of natural antioxidants during parboiling.
It has been found that properly processed extrusion-stabilized rice bran from rough rice can be safely stored for up to one year at .ltoreq.22.degree. C. in gas-permeable packaging. However, the maximum safe storage life of parboiled bran under the same conditions appears to be less than 3-4 months. There has been no known effective way to achieve a similar storage life for parboiled rice bran as elevated storage temperatures accelerate peroxide formation and the development of undesirable odors. There is a need for a stabilization method that not only prevents hydrolytic degradation, but also replaces the antioxidants that are lost in the parboiling process.
It would be advantageous to have a method of treating a parboiled rice bran so that it would be stable, i.e. have a peroxide value less than 20 meq/kg, for at least a 6-month period of time at ambient conditions.
It would additionally be advantageous to have a method of stabilizing a parboiled rice bran for use in a food product in which the food product containing the parboiled rice bran would maintain stability for at least a 6-month period of time at ambient conditions.
It would further be advantageous to have an animal feed comprising an amount of stabilized parboiled rice bran, in which the parboiled rice bran is stabilized by the addition of an edible acid having antioxidative properties to the parboiled rice bran to maintain the stability of the animal feed for at least a six month period of time at ambient conditions.